1. Field of the Invention
This invention relates to aircraft and, in particular, to an aircraft design system that permits an aircraft having a common fuselage to be fitted interchangeably with wings of various different planforms.
2. Description of the Prior Art
In conventional practice, the fuselage and the wing of an aircraft are designed to meet the requirements of the mission or task assigned to it. When that mission is changed, the wings and fuselage of the aircraft intended for the changed mission are redesigned accordingly. For example, aircraft designed for military tractical missions would have a certain type of wing, whereas aircraft designed for a military intercepter role would have a different wing planform although their fuselages could be substantially identical. Wings of an aircraft whose primary function is to carry a heavy load would have a different planform from the wings of aircraft of the lightly loaded high-speed type. Hence, for a given function, in conventional practice, the aircraft would have to be designed to fulfill that function. With the advent of modern propulsion systems, particularly the latest gas turbines, sufficient power is available to permit an aircraft to serve multiple roles effectively, but such is militated against by the unavailability of any efficient universal wing configuration. Thus, although a common fuselage is suitable for multiple roles, the lack of a suitable common wing rules out such multiple role with conventional aircraft. If it were possible to interchange wings routinely and efficiently, it would be feasible to use a common aircraft for multiple roles with all the advantages such commonality offers. However, in conventional practice, it is not possible to routinely prepare an aircraft for a specific mission merely by changing the wings to a configuration particularly suitable for that mission. Nor is it possible to use a particular aircraft fuselage to test various different wing configurations to thereby speed up the development time and reduce the costs of such programs.
The instant invention is a design system for aircraft and for various wing configurations to be used therewith that allows a routine change of wings to be made to suit the requirements of any particular role of the aircraft. The wing employed in the invention is of the type having half wings or semi-spans attached to a center section by which the wing is mounted on the aircraft fuselage. The center section is also termed a wing or torque box.
A critical factor in the design of aircraft is the provision of positive static longitudinal stability. An important consideration in attaining a required positive static longitudinal stability is the correct location of the mean aerodynamic chord (practically: mean geometric chord) of the wing relative to the center of gravity of the complete aircraft. The contribution of the wing itself to stability depends primarily upon the location of the aerodynamic center of the wing with respect to the aircraft center of gravity. Generally, the aerodynamic center or a.c. is defined as the point on the mean aerodynamic chord where the wing pitching moment coefficient does not vary with lift coefficient. In the complete aircraft, the combination of wing-fuselage-stabilizer (and/or canard) alters the aerodynamic center from the quarter-chord point of the wing alone (the approximate location of the a.c.) to a position of 2 or 3 percent of the mean aerodynamic chord further forward of the center of gravity.
The center of gravity moves in flight and with different loading conditions (i.e., fuel consumption and/or payload drop). It is arranged to be between closely controlled limits; the most forward being around 0.15 mean geometric chord of wing and the most aft about 0.35 mean geometric chord of wing. The limits bracket the aerodynamic center of the wing, which lies at the quarter chord (0.25 mean geometric chord of wing) point. In view of these considerations, a change from a wing of one planform to that of another conventionally entails a redesign of the fuselage, at least in the wing mounting area, so the different wing will not compromise positive static longitudinal stability.
I have found, however, that by designing the torque boxes of the wings that are to be used with a common fuselage such that they pick up the forward and aft spars of the wings and by providing a fixed standard pattern of mounting (hard) points on the fuselage and a mating standard pattern of mounting points on the torque box, wings as diverse as a forward swept and an aft swept can be mounted interchangeably on a common fuselage and still maintain an acceptable value for aircraft stability.
The invention herein thus contemplates interchangeable wings for aircraft. In this invention, the interchangeable wings each have semi-spans joined by a center section or torque box that fits into a cut-out in the fuselage. To permit different wing configurations to be used with a common fuselage, the torque boxes and the wing semi-spans are designed according to the invention such that the quarter chord point of the mean geometric chord of the wing, when installed in the fuselage cutout, is positioned in close proximity to the center of gravity of the aircraft. It is possible, therefore, to use aft-swept, forward-swept, or straight wings interchangeably with a common fuselage. The fuselage cut-out for the wings is defined at its forward and aft sides by structural bulkheads. The bulkheads are strengthened to pass into the fuselage structure the flight loads transmitted through the torque box by the wing. A set common pattern of wing mounting or hard points are provided on the fuselage bulkheads and on each of the torque boxes of the interchangeable wings. This provision of a set common pattern of wing mounting hard points permits the routine interchange of wings of various different planforms without adversely effecting the positive static longitudinal stability of the aircraft. This invention thus permits a wing configuration best suited for a particular mission to be routinely mounted on the aircraft.
It is, therefore, a principal object of the invention to provide an aircraft design system that permits wings having different planform configurations to be routinely interchanged without the re-design of the aircraft itself and/or the fuselage associated therewith being required.
Another object of the invention is to provide an aircraft design system that allows suitable wings to be interchangeably fitted to a common aircraft to obtain an optimum wing for the particular mission.
A still further object of the invention is to provide an aircraft design system that permits the retrofit of existing aircraft to permit the aircraft to accept different wing configurations interchangeably.
Yet another object of the invention is to provide an aircraft design system that permits the wing of an aircraft to be routinely changed to a configuration particularly suited for a particular task such that there can be a reduction in the inventory of types of aircraft required for operations.
A further object of the invention is to provide improved means for attaching wings of various designs to an aircraft, which means are of simple, uncomplicated construction permitting easy routine attachment and detachment of the wings, and which provide the required structural strength to pass wing loads into the fuselage structure of the aircraft.
Another object of the invention is to provide an aircraft design system which makes it feasible to fit wings of different planforms, such as forward swept, aft swept, or straight wings on a common aircraft fuselage without an unacceptable degradation in the aerodynamic balance of the aircraft.
Still another object of the invention is to provide standard fixed attachment means on the fuselage of the aircraft and on the wings to be used interchangeably therewith such that the required relationship of the aerodynamic center of the wing with respect to the center of gravity of the aircraft is maintained within acceptable limits during the routine interchange of wings of different planforms.
A still further object of the invention is to provide improved means of attaching wings of different planforms to an aircraft fuselage, which means are of rugged, uncomplicated design, which permit easy routine attachment and detachment of the wings, and which meet the required loads transmitted into the aircraft structure during all phases of flight.